The present invention generally relates to the treatment of contaminated liquid, such as wastewater. More particularly, the present invention defines an innovative apparatus and method for the mixing and electrolytic treatment of contaminated liquid which may contain electrolytes, dissolved solids, suspended solids, dissolved proteins, dissolved gasses, biological microorganisms and the like, which would advantageously be treated utilizing electrocoagulation, electrofloculation or electroflotation.
It is often desirable when treating contaminated fluid, such as wastewater, to add chemical additives to remove such contamination. For example, cationic or anionic chemicals may be added in predetermined amounts and which are intended to chemically bind with certain contaminants in order to facilitate their removal from the liquid. Such chemical additives, in the form of liquids, gases, or even solids, can be added to the contaminated liquid to be treated in order to alter pH, solution ionic strength, etc., which can be used beneficially to remove the contaminants.
In the past, such mixing methods and systems have comprised adding such chemical additives to a holding tank containing the contaminated liquid and mixing the chemical additives and liquid with an electric mixer having blades or the like. Other methods have included the injection of such chemical additives to a liquid stream as it enters into a hydrocyclone, or at a later stage before the previously treated contaminated liquid is introduced into a separation or holding tank. Most such methods and systems require rather complicated sub-systems or devices to adequately mix the contaminated liquid and intended chemical additives. Some of these devices and systems require power. Others require a relatively large footprint or space. The cost, complexity, and inconvenience of such systems and devices have sometimes provided disadvantages. Accordingly there is a need for a method for mixing chemical additives into contaminated liquids which does not require power, can be retrofitted into existing systems, and is of a relatively simple and inexpensive design.
It is well known by those skilled in the art, that the passage of electrical current through an electrically conductive liquid solution provides a useful means of affecting physical and chemical behavior in the treated liquid solution. Electrolysis is a general term describing a process in which chemical and physical reactions are stimulated by the passage of electrical current through a solution of an electrolyte. Of interest in the present invention is the electrolysis of aqueous electrolyte solutions.
An electrolytic apparatus as it applies to the scope of the present invention, is defined as a physical environment in which conductive electrodes are immersed in a solution containing an electrolyte. These electrodes are connected to an electrical power supply, such that a difference of electrical potential is present between the electrodes. The solution containing an electrolyte is in intimate physical contact with the electrodes, allowing a flow of electrons between the electrodes, and through the liquid solution. The relative amount of current flow through the solution is determined by the conductivity of the solution, relative to the electrode cross-sectional area.
The number of ions present in this solution determines the conductivity of an aqueous solution. The pH of the solution is the concentration of hydrogen ions present in the solution. The pH value defines the relative acidity or alkalinity of a solution. The pH also determines the “iso-electric point” in the solution, that is, the charge potential and sign of non-ionic suspended colloidal particles in the electrolyte solution. Solutions of electrolytes may contain acids, bases, or salts, made up of electrically charged particles, or “ions”. There is an equilibrium of positive and negative charges in such solutions called “cations” (+ charge) and “anions” (− charge). The splitting of compounds into cations and anions is referred to as ionization. Ionized solutions allow the transfer of electrons through the solution, when a sufficient difference of electrical potential is presented across such a solution. Thus, this mechanism allows the use of electrolytic cells comprising liquid in contact with solid conductive electrodes. A variety of electrochemical effects are stimulated by such an electron flow.
There are several methods of electrolysis which are well-known, including direct electrolysis, indirect electrolysis, electroflotation, electrocoagulation, and electroflouculation. Each will be briefly described below.
Direct Electrolysis is the electrochemical oxidation or reduction of pollutants occurring at the electrode surfaces without the involvement of other substances in the solution. Such treatment has been used for the destruction of organic pollutants such as phenols, aromatic amines, pesticides and the like, while inorganic compounds such as cynides have been desirably broken into non-toxic components. A major problem with apparatus for direct electrolysis is that the useful reaction zones are limited to the electrode interfacial areas, said areas including liquid layers no more than tens of molecules thick. Thus, a small fraction of the total liquid volume is subjected to direct electrochemical influence. The flowing liquid in such systems is isolated from treatment by boundary layer effects and diffusion time. The prior art tries to overcome these limitations by increasing electrode area in various ways, such as packed beds of conductive electrode particles, screens, surface treatments to increase surface area, and the like. Additionally, liquid flow rates are often restricted to improve the probability of the translation of microscopic volumes of the contaminated liquid into close proximity with electrode interfacial areas by thermodynamic means. Such flow rate restriction can lead to fouling, and low probability of useful treatment.
Indirect Electrolysis of pollutants relies on the electrochemical generation of reactive intermediate compounds, which then react with the targeted pollutant component. Powerful reactive agents such as hydroxyl radicals, hydrogen peroxide, ozone, hypochlorite or chlorine may be electrolytically generated. These reactive species then mix with the bulk liquid where secondary reactions occur. While these reagents, once produced, are useful in the treatment of wastewater, the overall efficiency of the treatment is still subject to the limitations of fouling, low flow rate, and poor mixing discussed in the previous paragraph.
Electroflotation utilizes dissociated oxygen and hydrogen gas volumes produced by the electrolytic process. Desirably small volumes of dissociated gas are often utilized to capture suspended hyrophobic contaminants into agglomerations of gas bubbles and contaminant particles, which then allow gravity separation of the contaminants. Secondary, non-ionic mechanisms, sometimes referred to as “field effects” can desirably effect suspended solids in waste streams. Thus, in addition to hydrophobic bubble attachment, there are colloidal effects that result from pH gradients in the liquid between electrodes, causing de-stabilization and agglomeration of colloidal suspensions.
Electrocoagulation is the destabilization of colloidal suspensions utilizing electrolytic effects. Secondary reactions resulting from electrolysis produce dissolved species, which can change pH and thus the iso-electric point of colloidal particles, breaking the suspension of colloids, and allowing their removal through subsequent attachment to charge sites on polymer additions.
Electrofloculation is an electrolytic process wherein particles in the solution are caused to be bridged or coalesce into groups as a result of intermediates generated by electrolysis, allowing their removal from the liquid stream using flotation, sedimentation, or filtration. There are many desirable electrolytic treatment methods, which may in themselves, or in combination with other treatment effect useful results.
Unfortunately, electrolytic treatment has in the past been limited in utility due to fundamental performance limitations of the prior art apparatuses and methods. Some of the problems with prior art electrolytic apparatus for wastewater treatment are:
1. Electrode fouling due to contaminants adhering to electrode surfaces, caused by low flow rates, low turbulence, and excessive electrode area. Electrode fouling lowers effective electrode area, and lowers operating efficiency.
2. Electrode blinding, due to low velocity of liquid flow, which allows the coalescence of undesirably large gas bubbles near electrodes, thus decreasing the effective cross sectional area of conductive electrolyte and lowering electrical efficiency.
3. Liquid passage plugging. Due to low liquid velocity typically employed in prior art apparatus, the low liquid velocity in the boundary layer of the electrode solid/liquid interface allows a gradual buildup of contaminant material. As the contaminants build up, the liquid path can be reduced to the extent that the liquid channel becomes plugged.
4. Low electrical efficiency in that the low liquid velocity utilized in the prior art to preclude plugging requires large electrode spacing, electrical energy utilization increases with commensurate decrease in performance. Desirable electrochemical effects of the process occur in close proximity to the electrodes, while current flow through the bulk liquid layer primarily effects undesirable heating due to IR losses in the bulk electrolyte.
5. Low mechanical mixing energy. In the liquid path of the prior art electrochemical devices, the liquid flow is primarily laminar. Turbulence generating devices utilized in some prior art devices, in an effort to improve mixing, tend to generate undesirable localized shear in the liquid, disturbing forming floccules, and lowering contaminant removal efficiency. Due to poor mixing, a large fraction of stream constituents may not be subjected to the desirable electro-chemical effects near an electrode. In electroflotation, bubbles may not optimally attach to hydrophobic waste particles. In electrocoagulation/electrofloculation, high shear may disrupt agglomerations of separable contaminants.
6. Complex mechanical configuration. Because prior art devices utilize excessive path lengths to overcome the inefficient electrode behavior, there may be multiple electrodes, packed beds, baffles, spacers, multiple seals and the like required. In addition, power distribution and bussing to individual electrodes is required. These arrangements are expensive to fabricate, assemble, and test.
7. High maintenance requirements. The complex nature of the prior art, as described above, require a higher level of maintenance, and increased downtime, increasing operating costs.
8. Large footprint requirements. The prior art being more complex and less efficient, requires more physical space for a given process throughput. This translates to a higher cost due to the ineffective use of real estate due to excessive electrode spacing.
Accordingly, there is a continuing need for an electrolyzing method and apparatus whose physical structure impedes the fouling of electrodes, and is otherwise less susceptible to plugging of liquid passage ways. What is also needed is an electrolytic apparatus with close electrode spacing, allowing higher electrical efficiency, while minimizing the undesirable blinding of the current pathway through the liquid solution. What is further needed is an electrolytic apparatus which provides desirable mixing energy throughout the liquid stream, without undesirable high shear. The apparatus should be simple and of low cost design with low maintenance requirements, and the smallest practical footprint size. The present invention fulfills these needs and provides other related advantages.